Infection of pupae of lepidopterans with live non-pathogenic bacteria induces the synthesis of a variety of antibacterial polypeptides secreted into the hemolymph. Previous studies have identified three main classes of antibacterial proteins namely lysozyme, cecropins and attacins [1]. Lysozyme [2,3] attacks the cell wall of gram-positive bacteria. The small (4-5 kDa), cationic cecropins [3,4] display a strong bactericidal effect against a variety of gram-positive and gram-negative bacteria. The attacins [5,6,7] (20 kDa) exist in two forms; one basic (pl=9) and one neutral pl=7) and the antibacterial effect is directed only against gram-negative bacteria.
Several forms of these antibacterial proteins have been found in various insect species. Peptides related to cecropins can be found not only in insects but also in vertebrates [8]. The same is true for the ubiquitous lysozymes. A protein related to the attacins, sarcotoxin IIA, has been found in the dipteran Sarcophaga [9].
Another class of antibacterial proteins from insects is the insect defensins [10]. They are characterised by an amino acid sequence of 38 to 43 amino acids containing six cysteines, forming three disulphide bridges. Different variants of insect defensins have been found in several insect species. Other related insect proteins are the diptericins with a molecular mass of 8.6 kDa that are effective against gram-negative bacteria [11] and the hemolins that belong to the immunoglobulin superfamily and are suggested to play a role in the regulation of cell adhesion during the cellular response to bacterial infections [12,13].
In addition to the antibacterial proteins from insects, there is also a number of antibacterial proteins isolated from mammalians e.g. the bactericidal/permeability increasing protein (BPI) [14,15] and the defensins [16]. The mammalian defensins differ structurally from insect defensins, although they have similar size and charge.